I. Field of the Invention
The present invention relates to data communication. More particularly, the present invention relates to a novel and improved method and apparatus for controlling transmissions of a communications system to increase efficiency and improve performance.
II. Description of the Related Art
Wireless communications systems are becoming more prevalent as the systems of choice for supporting a variety of communications applications. One such wireless communications system is a code division multiple access (CDMA) system that facilitates communication among a large number of system users. Other wireless communications systems include a time division multiple access (TDMA) system and a frequency division multiple access (FDMA) system. A Global System for Mobile (GSM) communications system is a TDMA-based system widely employed in Europe.
The spread spectrum modulation technique of CDMA has significant advantages over other modulation techniques for multiple access communications systems. The use of CDMA techniques in a multiple access communications system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d issued Feb. 13, 1990, and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d issued Apr. 7, 1992, both assigned to the assignee of the present invention and incorporated herein by reference.
CDMA systems are typically designed to conform to one or more CDMA standards. Examples of such CDMA standards include the xe2x80x9cTIA/EIA/IS-95-A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d and the xe2x80x9cTIA/EIA/IS-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d (collectively, the IS-95 standard), the TIA/EIA/IS-98-A, -B, and -C standards entitled xe2x80x9cRecommended Minimum Performance Standard for Dual-Mode Spread Spectrum Cellular and PCS Mobile Stations,xe2x80x9d and xe2x80x9cThe cdma2000 ITU-R RTT Candidate Submission,xe2x80x9d (hereinafter, the IS-2000 standard). New standards are continually proposed and adopted for use.
In a wireless communications system, communication between users is conducted through one or more base stations. A first user on one terminal (e.g., a remote station) communicates to a second user on a second terminal by transmitting data on an uplink to a base station. The base station receives the data and can route the data to another base station. The data is then transmitted on the downlink from the base station to the second terminal. The downlink refers to transmission from the base station to the terminal and the uplink refers to transmission from the terminal to the base station. In IS-95 systems, the uplink and the downlink are allocated separate frequencies.
In a wireless communications system, each transmitting source acts as potential interference to the receivers in the system. To combat the interference experienced by the terminals and base stations and to maintain a required level of performance, conventional TDMA and FDMA systems resort to frequency reuse techniques whereby not all time slots and frequency channels, respectively, are used in each cell. For example, a TDMA system may employ a 7-cell reuse pattern in which the total operating bandwidth W is divided into seven equal operating frequency bands (i.e., B =W/7) and each of the seven cells is assigned to one of the frequency bands. Thus, every seventh cell reuses the same frequency band. With reuse, the co-channel interference levels experienced in each cell are reduced relative to that if each cell is assigned the same frequency band. However, reuse patterns of more than one cell (such as the 7-cell reuse pattern for a conventional TDMA system) represent inefficient use of the available resources since each cell is allocated and able to use only a portion of the total operating bandwidth.
A CDMA system is capable of operating with a 1-cell reuse pattern (i.e., adjacent cells can use the same operating bandwidth). However, the CDMA system is designed to carry voice data having a low data rate (e.g., 32 kbps or less). Using code division spread spectrum, the low-rate data is spread over a wide (e.g., 1.2288 MHz) bandwidth. Because of the large spreading factor, the transmitted signal can be received at a low or negative carrier-to-noise-plus-interference (C/I) level, despread into a coherent signal, and processed. The CDMA system is not designed for data transmission at high data rates.
Given the ever-growing demand for wireless communication, method and apparatus that can support data transmission at high data rates and allow for better utilization of the available resources to increase efficiency and improve performance are highly desirable.
The present invention provides techniques for controlling transmissions of a communications system to increase efficiency and improve performance. A communications system is typically required to satisfy a particular coverage criteria typically defined by a specified minimum average bit rate being achieved for a prescribed percentage (e.g., 99.99%) of the time and/or a particular percentage (e.g., 99%) of the users with received signal levels exceeding a particular C/I threshold. The coverage requirement is often influenced by a small percentage of disadvantage users that experience excessive interference from a small number of interfering sources.
The invention recognizes this fact, and provides various techniques that attempt to assist disadvantage users when and where possible to achieve the coverage requirement. In accordance with certain aspects of the invention, each cell in the system can be designed to operate in accordance with a set of back-off factors that identify the reductions in peak transmit power levels for the channels associated with the back-off factors. The back-off factors are defined to provide the required power to a large percentage of the users while reducing the amount of interference.
In accordance with other aspects of the invention, the cells operate using an adaptive reuse scheme that allows the cells to efficiently allocate and reallocate the system resources to reflect changes in the communications system. A reuse scheme is initially defined and resources are allocated to the cells. During operation, changes in the operating conditions of the system are detected and the reuse scheme is redefined as necessary based on the detected changes. For example, the loading conditions of the cells can be detected, and the resources can be reallocated and/or the reuse scheme can be redefined based on the detected loading conditions.
In accordance with certain other aspects of the invention, techniques are provided to efficiently schedule data transmissions and to assign channels to users. Data transmissions can be scheduled based on user priorities, some fairness criteria, system requirements, and other factors. The data transmissions to the users are assigned to available channels based on a number of channel assignment schemes. Channel metrics are also provided, which can be used to prioritize users and for channel assignments. These various aspects of the invention are described in further detail below.
A specific embodiment of the invention provides a method for controlling transmissions in a communications system. In accordance with the method, the available system resources are first partitioned into a number of channels. One or more characteristics of the communications system are determined and a set of back-off factors is defined for the channels based on the determined system characteristics. Each channel is associated with a respective back-off factor, ranging from zero to one, that identifies a reduction from the peak transmit power level. Data transmissions are transmitted on the channels at power levels determined based on the back-off factors. One or more channels are typically associated with a back-off factor of one, representative of full transmit power, and remaining channels are typically associated with back-off factors of less than one.
The available system resources can be partitioned into a number of time division multiplexed (TDM) time slots, frequency division multiplexed (FDM) channels, or code division multiple access (CDMA) channels. The channels then correspond to defined sets of TDM time slots, FDM channels, CDMA channels, or a combination thereof
The back-off factors can be defined based on the carrier-to-noise-plus-interference (C/I) characterization of the receiver units in the communications system, the loading probabilities, the required outage probabilities, the set points (i.e., required C/I), or other characteristics or parameters of the system.
The back-off factor for each channel can be adjusted based on the estimated required transmit power level for the channel. The required transmit power can be estimated based on estimated or measured C/I, frame erasure rate (FER), outage probabilities, and others. The back-off factors can also be adjusted based on changes in the communication system (e.g., changes in user characterization, loading, C/I requirements, and so on). One or more back-off factors can be reduced (or possibly set to zero) for selected time durations to reduce interference on the associated channels. The back-off factor for a highly degraded channel having excessively poor frame erasure rate (FER) and/or outage probability may also be set to zero.
For a multi-cell system, a set of back-off factors can be defined for each cell based on the determined characteristics of the cell. The back-off factors for a particular cell are approximately staggered from those of neighboring cells in the reuse pattern to reduce interference. A cell may request other cell(s) to temporarily reduce or shut down their transmit powers so that a particular disadvantage user can be served. If a cell receives multiple requests for power reduction, the cell can apply the largest requested power reduction. The power reduction can be applied in various manners (e.g., in defined steps, by specified amounts, and so on, at designated time intervals). The back-off factors assigned to the cells may also be modified or adjusted to reduce the amount of co-channel interference. Each cell may also be designated time intervals in which data transmissions are allowed. Also, the back-off factors can associated with sectors in a sectorized cell (or with any directed transmission to a particular geographic area).
Another specific embodiment of the invention provides a method for operating a number of transmitter units in a wireless communications system. In accordance with the method, the available system resources are first partitioned into a number of channels. A reuse pattern that includes a number of cells is then defined for the system. One or more characteristics for each cell in the reuse pattern are determined, and a set of channels is allocated to each cell in the reuse pattern based on the determined cell characteristics. The cell characteristics are continually determined and new sets of channels can be allocated to reflect changes in the communications system.
Each cell in the reuse pattern is typically allocated a set of channels for transmission at full power level and may further be allocated one or more channels for transmission at reduced power levels. The channel allocation is typically dependent on a number of factors such as, for example, the number of available channels, the number of cells in the reuse pattern, the user characterization, the loading conditions in the cells, and others. In some implementations, a cell may transmit on a non-allocated channel when additional capacity is needed. The non-allocated channel can be selected based on, for example, its estimated performance, probability of occupancy by other cells, outage probability, and so on. One or more channels can be reserved for transmissions by a particular cell for a particular time duration.
Yet another specific embodiment of the invention covers a method for providing data transmissions to a number of receiver units in a communications system. In accordance with the method, a first set of parameters to be used for scheduling data transmissions is updated, and the data transmissions to the users are prioritized and assigned to available channels based on their priorities. A second set of parameters to be used for transmitting is updated, and the data transmissions are transmitted on the assigned channels to the receiver units using the updated second set of parameters. The performance (e.g., FER) of the data transmissions can be measured and the transmit power levels and/or data rates for the data transmissions can be adjusted accordingly based on the measured performance.
The first set of parameters may include, for example, channel occupancy probabilities, loading probabilities, characterization of C/I of the receiver units, or back-off factors, or a combination thereof. To perform the prioritization, channel metrics may be computed for the available channels for each receiver unit using the updated first set of parameters. The channel metrics can relate to the (e.g., realizable or actual) cumulative throughput for the receiver units, outage probabilities, expected C/I, or some other measures, and may further be reflective of expected interference from interfering sources.
The data transmissions are assigned priorities based on the computed channel metrics, and the priorities may be modified based on, for example, transmission delays. The data transmissions can be assigned to the available channels based on the assigned priorities and the computed channel metrics. In some implementations, successively lower priority data transmissions are assigned to the available channels, starting with the highest priority data transmission. In some other implementation, successively smaller load data transmissions are assigned to the available channels, starting with the highest load. The channel assignment can also attempt to approximately equalize the data rates for some of the data transmissions.
Yet another specific embodiment of the invention covers a method for providing data transmissions on a number of channels to a number of receiver units in a communications system. In accordance with the method, a set of back-off factors are defined for the channels, where the back-off factors identify a reduction from peak transmit power level for the channels. Data transmissions are assigned to the channels, and the required transmit power levels are determined for the data transmissions. The back-off factors are adjusted in accordance with the required transmit power levels, and the data transmissions are transmitted on the channels in accordance with the adjusted back-off factors. Various aspects to define and adjust the back-off factors, schedule the data transmissions, and assign channels described above can be applied.
Yet another specific embodiment of the invention provides a transmitter unit that includes a system data processor, one or more modulators, and one or more antennas. The system data processor receives and partitions an input data stream into a number of channel data streams and to process the channel data streams. The modulator(s) receive and modulate the processed channel data streams to generate one or more modulated signals that comprise a number of data transmissions to be transmitted on a number of channels to a number of receiver units. Each channel is associated with a respective back-off factor, ranging from zero to one, that identifies a reduction from the peak transmit power level. The antenna(s) receive and transmit the modulated signal(s).
The back-off factors for the channels are defined based on various system characteristics such as, for example, the C/I characterization or the loading of the system. The modulator(s) can be designed to implement orthogonal frequency division multiplexing (OFDM) modulation.
Yet another specific embodiment of the invention provides a receiver unit that includes one or more antennas, one or more demodulators, and a data processor. The antenna(s) receive one or more modulated signals that have been generated and transmitted by (1) partitioning an input data stream into a number of channel data streams, (2) processing and modulating the channel data streams to generate one or more modulated signals that comprise a number of data transmissions to be transmitted on a number of channels, and (3) adjusting power levels of the data transmissions in accordance with a set of back-off factors associated with the channels. The demodulator(s) receive and demodulate the modulated signal(s) to generate one or more demodulated symbol streams, and the data processor receives and processes the demodulated symbol stream(s) to generate output data.
Various other embodiments of the transmitter and receiver units are also provided, as described below. Moreover, various aspects, features, and embodiments of the communications system are also described below.